Conventional X-ray tubes substantially comprise a vacuum chamber with a housing enclosing a cathode and an anode. The cathode forms the negative electrode, which emits electrons toward the positive anode. The electrons are attracted from the anode and strongly accelerated by an electrical field between the anode and cathode. The anode is typically made of a metal, for example tungsten, molybdenum or palladium. When the electrons bombard the anode, their energy is for the most part converted into heat. Only a fraction of the kinetic energy can be converted into X-ray photons, which are emitted by the anode in the form of an X-ray beam. The X-ray beam generated in this way exits the vacuum chamber through a radiation-permeable window made of a material with a low atomic number.
Applications in industrial and medical imaging and for therapeutic treatment are nowadays unimaginable without X-ray devices. X-ray devices are also used to treat vascular diseases inside patients' bodies. For this, X-ray devices must be miniaturized sufficiently to enable them to be introduced into a patient's vessels.
An X-ray tube of this kind is disclosed in DE 198 29 444 A1. The X-ray device is preferably arranged at the distal end of a catheter. The X-ray tube has a vacuum housing equipped with a cylindrical housing section the inside wall of which is coated with a target material. A cylindrical isocentrically arranged field emission cathode extending along the longitudinal axis is located in the vacuum housing and emits electrons radially outward in the direction of the target material for the generation of X-rays. When the electrons hit the target material, X-rays are generated which penetrate the vacuum housing. The X-ray tube can be designed small enough to enable the treatment of even coronary vessels.
For some time now, carbon nanotubes have also been used to build cathodes for multi-beam X-ray tubes. For example, PCT application WO 2004/110111 A2 discloses an X-ray tube of this kind. The multi-beam X-ray tube comprises a stationary field emission cathode and an anode facing the cathode. The cathode comprises a plurality of stationary, individually controllable electron-emitting carbon nanotubes disposed in a predetermined pattern on the cathode. The anode comprises a plurality of focal points disposed in a predetermined pattern corresponding to the pattern of the carbon nanotubes. A vacuum chamber encloses the anode and cathode.
The solution disclosed in WO 2004/110111 A2 offers many advantages compared to conventional thermionic sources of X-radiation. It eliminates the anode's heating element, operates at room temperature, generates pulsed X-rays with a high repetition rate and generates a plurality of beams with different focal points.